Non-contact, electric field measurements have been a challenge due to the need for constructing low-noise amplifiers with extremely high input impedance (>100 fF∥10 TΩ) and low noise (>0.1 fA/Hz1/2). Prior art solid-state electric field sensors, such as those described in U.S. Pat. No. 6,686,800, US 2011/0043225, and U.S. Pat. No. 7,439,746, used for both free-space and biological applications, have relied on commercially available ‘discrete’ operational amplifiers or instrumentation amplifiers. One example is the TI INA116, from Texas Instruments, Dallas, Tex., which has an input impedance typically on the order of 2-5 pF∥1 TΩ and current noise levels of (0.1-0.5 fA/Hz1/2).
For electric field sensors, it is typically desirable to maintain a high input impedance, which is dictated by the circuit elements that are connected to the sensor input. Any circuit element having a conductance or capacitance and connected to the sensor input necessarily degrades the input impedance. However, there are always circuit elements connected as part of the circuit's normal operation (e.g., an amplifying transistor, biasing resistor, and shield) and other parasitic byproducts (e.g., neighboring electrical connections to the input). Such circuit elements include at least one terminal connected to the sensor input and one or more terminals that are connected elsewhere. Prior art designs typically use an active shield, a well-known technique, to raise the sensor input impedance by driving the other terminals of the circuit elements such that the potential difference across any circuit element to the sensor input is zero. For purposes of the description herein, the terms “active shielding”, “guarding” and “bootstrapping” are defined according to their common definitions as would be understood by one of skill in the art.
Although active shielding has been effective in the prior art for minimizing the input capacitance on the packaging and circuit board level, its efficacy is reduced for reducing the internal capacitance of a discrete amplifier. Commercial discrete amplifiers have, at the input, at least an electrostatic discharge (ESD) protection structure, packaging parasitic and device parasitic capacitance that are completely inaccessible and contribute at least 2-10 pF of input capacitance. Additionally, attempts at implementing a high impedance amplifier using discrete components (e.g., transistors, resistors, capacitors) with bootstrapping have become difficult, if not impossible, due to the lack of suitable discrete FET parts with appropriate specifications (e.g., low gate leakage for JFETs, and low leakage ESD for MO SFETs).
Overcoming the internal input capacitance within the discrete amplifier has required the use of a positive feedback network that comprises of a second amplifier, with gain greater than unity, driving a neutralization capacitor, a technique known to a person skilled in the art. Implementation is difficult due to the need for manual calibration and tuning. In addition, the use of neutralization is additionally imprecise due to the non-linear input capacitance (e.g., P-N junction capacitance of protection diodes) of a typical discrete amplifier, which may vary across operating conditions making the entire process inherently imprecise and difficult to manufacture. Finally, the neutralization amplifier often requires an additional power supply with a greater voltage range than that supplied to the sensor amplifier.
Other prior art has shown the possibility of bootstrapping a discrete amplifier's power supply to avoid the need for neutralization (U.S. Pat. No. 7,439,746). This method is effective but has an additional set of limitations, including that: 1) it requires a careful selection of components including the specific discrete amplifier part since this mode necessarily operates the part outside of its recommended usage; 2) a large voltage minimum supply range is necessary (>5-10V) since the supply must accommodate both the primary amplifier (3V) plus an additional overhead required to operate the power supply bootstrap circuit (˜3-5V); and 3) stable operation is difficult to achieve due to the multiple feedback paths involved in bootstrapping of all the ports in a discrete amplifier.
Additionally, DC biasing the sensor input has been difficult due to the need for high resistance (>100 GΩ), low-noise (<0.1 fA/Hz1/2), low-leakage (<20 fA), elements. Prior art has likewise used discrete circuit components, typically resistors or diodes, with or without bootstrapping, to supply the amplifier's input bias current and stabilize the DC potential of the sensor's input. The use of discrete components is subject to the following disadvantages: 1) low noise biasing resistors (<0.1 fA/Hz1/2 current noise) are not commercially available and can be only implemented at great cost; 2) other input bias techniques, such as diodes, can provide lower noise but add additional leakage and capacitance to the input; and 3) discrete components add more parasitic capacitance and leakage than integrated versions of the same, adding noise to the sensor.
In view of the failure of the prior art to overcome the disadvantages described in the foregoing, the need remains for a sensor with ultra-high input impedance that is suitable for sensing of electric fields.